A printed circuit is used in information terminal apparatuses, such as a personal computer and a portable phone. A laminated sheet for use in a printed wiring board constituting the printed circuit is prepared by impregnating a base material with an electrically insulating resin and curing the resin until it is in a B-stage to form a prepreg, and stacking the prepregs on one another and bonding them together. When forming the printed wiring board by a subtractive method, a metal-clad laminated sheet is used. The metal-clad laminated sheet is prepared by placing a metal foil, such as a copper foil, on the surface (one surface or both surfaces) of a prepreg and pressing them together while heating.
In recent years, as electronic communication apparatuses, e.g., information terminal apparatuses, such as a personal computer and a portable phone, are being spread, various demands have been made on the performance of the printed wiring board mounted on these apparatuses. Therefore, a prepreg and a prepreg material are requested to be improved in the properties in relation to the above demands. For example, electronic communication apparatuses are further reduced in weight, thickness, length, and size and further increased in speed and frequency. The technique for mounting chips is changed from a through-hole mount type to a surface mount type, and further to an area array type, typically ball grid array (BGA) using a plastic substrate. Recently, chip mounting is generally conducted by thermosonic wire bonding. A substrate on which chips are mounted is exposed to temperatures as high as 150° C. or more during the chip mounting, and therefore the substrate (substrate material) is required to have a certain heat resistance.
In addition, performance such that the mounted chip can be replaced, so-called repairability is demanded. In the replacement of chips, heat equivalent to that used in the above chip mounting is applied to the chip and the chip is removed, and then heat is applied again to the chip when remounting it. In this case, conventional insulating resins have a problem in that peeling may occur between a fiber base material and the resin. Therefore, the substrate required to have repairability is also required to have a thermal shock resistance including a resistance in thermal cycling at high temperatures.
Further, in accordance with the increase of the processing speed, the number of I/O in MPU is increased, and therefore the number of terminals connected thereto by wire bonding is increased and the terminal width is reduced. Thus, for improving the bonding strength between the prepreg and the metal foil on which circuits are formed and for forming even thinner wiring, a metal foil having a surface further finely roughened is desired.
In addition, as the frequency of the signal becomes higher, the circuit conductor is required to have surface smoothness. Magnetic lines of force are generated around a current which flows a conductor, and interference of the magnetic lines of force is most remarkable in the central portion of the conductor, and hence the current is concentrated on the periphery and end portions of the conductor. This is called a skin effect, and, the higher the frequency of the signal, the more remarkable the skin effect. It is considered that, when the surface of the conductor is smooth, the increase of the resistance due to the skin effect can be suppressed. However, the bonding strength between a conventional electrically insulating resin and a metal foil largely depends on the anchoring effect obtained mainly by the rough profile of the surface of the metal foil. For this reason, it has been difficult to achieve both the increase of the signal frequency and the improvement of the bonding strength.
As the electrically insulating resin, thermosetting resins, such as an epoxy resin, a phenolic resin, a polyimide resin, and a bismaleimide-triazine resin, have been generally used, and thermoplastic resins, such as a fluororesin and a polyphenylene ether resin, are also used. As the base material, glass, paper, and synthetic fiber are used. However, an epoxy resin has poor heat resistance and it is peeled off easily at the interface between glass and the resin due to heat, leading to a problem of the adhesion between the base material and the impregnating resin. Further, a polyimide resin has disadvantages in that the moldability and adhesion properties are poor and the curing temperature is high (about 250 to 400° C.). For example, the bonding strength of a polyamide-imide resin to the gloss surface of a metal, such as stainless steel, an iron-42 nickel alloy, or copper, or glass is lower than the practical level.
For example, Japanese Unexamined Patent Publication No. 181511/1991 has a description concerning a method for producing a polyamide-imide polymer in which (1) an aromatic tricarboxylic anhydride is reacted with a diamine having an ether linkage, and then the resultant product is reacted with (2) a diisocyanate. However, the polyamide-imide polymer obtained by such a method is likely to have a branched structure, and further a structure is formed in which the basic structures comprising an aromatic unit, an amide bond, and an imide ring are randomly arranged, and therefore it has a disadvantage in that the heat resistance and mechanical properties are unsatisfactory.
As mentioned above, with respect to the printed wiring board and the prepreg and the impregnating resin which are materials for the printed wiring board, it is desired that the heat resistance, the weathering resistance (thermal shock resistance indicated by heat cycle), and the adhesion properties which do not depend on the state of the rough profile of the surface of a metal foil are excellent.
An object of the present invention is to solve the above problems and to provide a prepreg which is advantageous not only in that it has excellent heat resistance and can be bonded to a smooth metal foil, but also in that the adhesion between the metal foil and the prepreg is excellent, and an insulating substrate, a metal-clad laminated sheet, and a printed wiring board as the use of the prepreg.